1. Field Of The Invention
This invention relates to a process for the selective removal of sulfur dioxide and mixtures of sulfur dioxide and at least one of sulfur trioxide and nitrogen oxide from effluent waste gas streams containing a preponderance of carbon dioxide and in which the absorption of carbon dioxide from these effluents is kept to a minimum. Preferably, the sulfur compounds removed from the waste gas streams are then converted to useful materials such as elemental sulfur or sulfuric acid.
2. Prior Art
The economic removal of sulfur dioxide from gaseous industrial effluents is a difficult problem which has defied satisfactory solution to the present time, but which has become a subject of intense scrutiny as concern for atmospheric pollution and ecological contamination has increased. The difficulty of economic removal has arisen essentially from the fact that the sulfur-containing gases are generally present in very low concentrations in an enormously large stream of industrial effluent gas such as from combustion waste gases, or stack gases of electric power plants, off-gases from incinerators and from other chemical and petroleum refinery plants.
A number of solutions to this problem are known to the art for the purification of gaseous streams contaminated with SO.sub.2 and SO.sub.3. Among the most widely used processes are those employing lime or limestone, in solution or slurry, to yield the corresponding sulfite, bisulfite and sulfate salts. Gas treating systems of this type utilize reagents in greater than stoichiometric amounts and lead to a formidable solid waste disposal problem, which is quite objectionable. Disposal of the waste product in an economic and ecologically satisfactory manner has plagued such noncyclic operations. Thus, in a scrubbing process in which lime is used and calcium salts are produced, the cost and volumetric space required in the disposal of the large amounts of waste impart a severe economic impediment to the feasibility of such nonregenerative methods of abatement.
There are significant ecological advantages to cyclic processes utilizing scrubbing solutions which can be regenerated by heating and used over again, to yield concentrated SO.sub.2 for conversion to an easily disposed-of form, such as to sulfur or sulfuric acid. These cyclic operations, utilize both inorganic and organic type solvents. Among the inorganic absorbents most widely used, are aqueous solutions of ammonia, sodium hydroxide and other alkali metal hydroxides or carbonates, to yield the corresponding sulfite, bisulfite and sulfate salts. These scrubbing reagents, however, are not without their problems and disadvantages. For example, aqueous ammonia is quite volatile and its use could result in significant solvent losses to the atmosphere, as well as to air pollution problems. The use of caustic solution under certain operating conditions, as outlined in U.S. Pat. Nos. 3,719,742 and 3,790,660 to Wellman-Lord Corporation, lead to an aqueous slurry in the desorbing zone which is difficult to handle. In addition, these processes have been plagued with inefficiencies typified by a high stripping steam requirement.
These and other problems have been partly resolved by the use of high-boiling, water-soluble organic solvents. The use of alkanolamines, such as aqueous solutions of triethanolamine, has been reported as a highly efficient method for absorbing SO.sub.2 from waste gases in a cycle in which the triethanolamine solvent contacts the waste gas to absorb the sulfur oxides and is thereafter stripped by heat to release the sulfur dioxide in a concentrated form. The stripped solvent is then recycled back to the absorber for further use. This type of system, using aqueous organic solvents for the selective removal of SO.sub.2 is disclosed in U.S. Pat. No. 3,904,735. This reference specifically teaches that monethanolamine (MEA) is unsatisfactory for the following three factors:
(a) MEA solvent, due to its high basicity, does not selectively absorb SO.sub.2 readily, without absorbing appreciable quantities of CO.sub.2 ;
(b) High evaporative losses of the solvent due to its high volatility;
(c) A high rate of oxidation of SO.sub.2 to SO.sub.3 takes place in MEA solvent system, because of the oxygen present in the flue gases. The SO.sub.3 is not normally strippable from the solvent and results in loss of MEA absorbent capacity as it accumulates.